Now, you want to see this fabulous transition that you've created. If you hit the space bar to Play your Timeline, it won't show up. This is because Premiere needs to merge the two clips into one to complete the transition effect. This is called rendering and is required for many video effects in Premiere, including titles.
The section of your timeline that needs to be rendered is indicated by a thin red line at the top of the Timeline window above the transition.
So, hold down Control-Alt and click on the Work Area bar above the time code until a little red arrow appears. Move it to cover the first two clips.
From the menu bar, choose Timeline > Render Work Area. Premiere creates new frames (enough to cover the duration of the transition) and saves them to your hard drive. Notice that the thin red line above the transition turns green. That means it's rendered, or saved to the hard drive. Rendering only has to be done once, unless you make changes.
If you don't like that transition, go into the Transition palette and choose another. Simply drag it over the Cross Dissolve. Note that the green line turns red again, which means you have to render it before you can see it.